1. Field of the Invention
The present invention relates to an autofocus method of precisely focusing a beam even onto a flat specimen having no edges in a scanning charged-particle beam instrument.
2. Description of Related Art
Autofocusing is achieved in a scanning electron microscope by scanning an electron beam over a specimen region, deriving secondary electrons from the region, evaluating variations in the brightness produced at successive scan positions, and feeding the result of the evaluation back to control the focal distance of the focusing lens (e.g., objective lens) of the scanning electron microscope.
FIGS. 9A and 9B illustrate electron beam scanning performed when an automatic focusing operation is performed. FIG. 9A shows one example of vector scanning, and FIG. 9B shows one example of raster scanning.
The graph of FIG. 10 shows the brightness of a secondary electron signal obtained at each electron beam scan position. The brightness is plotted on the vertical axis. The scan position is plotted on the horizontal axis.
Such scanning of the electron beam over a specimen region is performed at various focus values. Whenever the value is varied, the relationship between the beam scan position and the brightness signal is found and acquired. The signal is differentiated at each focus value. A focus value resulting in a maximum sum of values obtained by differentiation is taken as an optimum focus value.
FIG. 11 is a block diagram of an example of configuration of an autofocus system for use in a scanning electron microscope. The microscope has an electron gun 1 emitting an electron beam 2, which, in turn, is focused to a small diameter onto a specimen 5 by an objective lens 4. A desired region of the specimen 5 is scanned by the beam, using a deflector 3. At this time, secondary electrons are produced from the specimen 5 and detected by a secondary electron detector 6. The output signal from the detector 6 corresponding to the detected secondary electrons is converted into digital image data by an A/D converter (ADC) 7.
A microcomputer 10 is made up of an evaluation portion 11, an arithmetic-and-control portion 12, and a scanning signal-generating portion 13. The output signal from the scanning signal-generating portion 13 is sent to a deflection driver circuit 9, which, in turn, drives the deflector 3. Therefore, the electron beam can perform two-dimensional scanning over the specimen 5.
The output signal from the A/D converter 7 is entered into the evaluation portion 11, where the signal is evaluated. The result of the evaluation is sent to the arithmetic-and-control portion 12, which calculates an amount of feedback based on the result of the evaluation made by the evaluation portion 11 and outputs the calculated amount (value). The output signal from the arithmetic-and-control portion 12 is input to a focal distance driver circuit 8.
In operation, the electron beam 2 passes through the objective lens 4 and hits the specimen 5. The objective lens 4 is excited with an excitation current based on the output signal from the focal distance driver circuit 8. At this time, secondary electrons produced from the specimen 5 are detected by the secondary electron detector 6. The output signal from the detector 6 indicative of the detected secondary electrons is converted into digital image data by the A/D converter 7 and applied to the evaluation portion 11.
The evaluation portion 11 is measuring the detector output signal indicative of the detected secondary electrons. The measured value is supplied to the arithmetic-and-control portion 12. The arithmetic-and-control portion 12 drives the focal distance driver circuit 8 in such a way that the objective lens 4 is excited with an excitation current which brings the beam 2 into sharp focus onto the specimen 5. In this way, the automatic focusing operation is carried out.
A known technique for this kind of system is described, for example, in JP1173903. In particular, an automatic focusing operation is performed using a whole field of image. Then, the detector output signal indicative of the whole field of image is stored in a memory. The stored signal is read out. The signal is accumulated in every subregion obtained by division by a data accumulation unit. The automatic focusing operation is repeated in areas where the accumulation values are high among the subregions.
Another known technique is described, for example, in JP5114378. Specifically, a deflection coil is driven with a vertical scan signal to which a horizontal scan signal and a sawtoothed wave are added. During the period of one horizontal scan signal, the scanning is repeatedly done at short intervals in the vertical direction, using the sawtoothed wave. Secondary electrons produced from the specimen by the scanning are detected, whereby an automatic focusing operation is performed.
In order to operate the autofocus system shown in FIG. 11, the brightness must be varied in a corresponding manner to the beam scan position as shown in FIG. 10. However, when the electron beam is scanned over the specimen, if the line on the specimen hit by the beam happens to have no structural object thereon, if the beam deviates from the scanned line due to fine dust, or if the surface of the specimen can be regarded as a flat plate, the brightness is constant with regard to the beam scan position as shown in FIG. 12. Consequently, it is unlikely that the brightness varies with the beam scan position. Hence, it is impossible to perform signal processing effectively. There is the problem that it is impossible to bring the beam into focus.